Black tourmaline is an amazing stone. I always have a little on me, in my pocket and a big one at my office (it is very good against computer radiations) and one at home (a huge one I had the luck to find at a stone convention) that I have placed near my electricity panel.
One advice : it doesn't need to be cleaned in water as often as other stones and, most important, don't purify it with the sun rays, Tourmaline hates that ;-)
this was written in a book I trust 100% and confirmed frankly by my pendulum.